2 files changed, 10 insertions, 4 deletions
diff --git a/activerecord/lib/active_record/associations/builder/has_many.rb b/activerecord/lib/active_record/associations/builder/has_many.rb
index 1c1b47bd56..eda3b74528 100644
--- a/activerecord/lib/active_record/associations/builder/has_many.rb
+++ b/activerecord/lib/active_record/associations/builder/has_many.rb
@@ -5,7 +5,7 @@ module ActiveRecord::Associations::Builder
     end
 
     def self.valid_options(options)
-      super + [:primary_key, :dependent, :as, :through, :source, :source_type, :inverse_of, :counter_cache, :join_table, :foreign_type]
+      super + [:primary_key, :dependent, :as, :through, :source, :source_type, :inverse_of, :counter_cache, :join_table, :foreign_type, :index_errors]
     end
 
     def self.valid_dependent_options
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index 0792d19c3e..3c0a39d10f 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -141,6 +141,8 @@ module ActiveRecord
 
     included do
       Associations::Builder::Association.extensions << AssociationBuilderExtension
+      mattr_accessor :index_nested_attribute_errors, instance_writer: false
+      self.index_nested_attribute_errors = false
     end
 
     module ClassMethods
@@ -315,7 +317,7 @@ module ActiveRecord
       def validate_collection_association(reflection)
         if association = association_instance_get(reflection.name)
           if records = associated_records_to_validate_or_save(association, new_record?, reflection.options[:autosave])
-            records.each { |record| association_valid?(reflection, record) }
+            records.each_with_index { |record, index| association_valid?(reflection, record, index) }
           end
         end
       end
@@ -323,14 +325,18 @@ module ActiveRecord
       # Returns whether or not the association is valid and applies any errors to
       # the parent, <tt>self</tt>, if it wasn't. Skips any <tt>:autosave</tt>
       # enabled records if they're marked_for_destruction? or destroyed.
-      def association_valid?(reflection, record)
+      def association_valid?(reflection, record, index=nil)
         return true if record.destroyed? || record.marked_for_destruction?
 
         validation_context = self.validation_context unless [:create, :update].include?(self.validation_context)
         unless valid = record.valid?(validation_context)
           if reflection.options[:autosave]
             record.errors.each do |attribute, message|
-              attribute = "#{reflection.name}.#{attribute}"
+              if index.nil? || (!reflection.options[:index_errors] && !ActiveRecord::Base.index_nested_attribute_errors)
+                attribute = "#{reflection.name}.#{attribute}"
+              else
+                attribute = "#{reflection.name}[#{index}].#{attribute}"
+              end
               errors[attribute] << message
               errors[attribute].uniq!
             end